Draw pictures, anything really. Let your creative juices flow! :)
Place on paper and PAINT PAINT PAINT!
Tip: Try not to think about what's going to happen, just focus on painting the paper and try to ignore the "mess" your making on the card!
Leave to dry, and you should end up with this.
Find some funky card. Cut them both so they fit nicely, then glue!
PS- Try and do it better than me ;)